I'm planning to get a set tyres for my z4 e86 2008, coupe 3.0si, stock M sport suspension, recently purchase. I don't have any experience on a race track andI don't want top performance, since I won't race the car, but I only want decent tyres that can keep the car predictable at the limit (which depends on the tire selection). I was thinking of some weid idea, of fitting toyo proxxes r888 225/40/r18 on front wheels to acheive maximum grip and avoid understeering, and michellin pilot sport 4, 255/35/r18 on the rear. I've noticed the recommended tyres have also different heights, if I'm not mistaken..
What could go wrong ?

To keep the car predictable at speed you don't want to completely eliminate under steer. If you have under steer and you come into a corner a bit too fast you'll go a bit wide on the turn and maybe drop a couple wheels into the turf. With over steer if you go into a turn too fast and the rear comes around on you, there's a good chance you're going into the wall/fence.

You may be able to moderate under steer a bit with your indicated tire selection, but there's more factors than just treadwear rating that play into tire performance. Race/track/autocross dedicated tires don't develop full grip until they come up to operating temperature, which is hotter than street tires. They're notoriously slippery until they get up to temp, and with the combo you've indicated you may have really significant under steer until the Toyos get warm enough (assuming you can get them to optimum temp). Street tires are designed to have good grip at moderate temperatures, but get very slippery and greasy when they get too hot.

With the combo you're looking at you could have a situation where the first few laps the car will under steer severely, and then as the Toyos get up to temp and you start to generate front grip, the PS4s may be getting to over temp and the rear end gets very loose. The end result could be a car that won't turn initially and then transitions to snap over steer.

My opinion (and you're certain to get plenty on this topic) is that you'd be better off keeping same tire type on both ends of the car, go wider on the front, maximize your front camber by pulling the factory alignment pins, and set front toe to zero. You can run a 245 series tire on your 8" front rims, and potentially even a 255 series tire if it has reasonably flexible sidewalls to tolerate the added pinch fit. I've run 255/35 x 18 on the front and 275/35 x 18 on the rear on the OEM wheels, and currently I'm running 245/40 x 18 front and 265/35 in the rear. For track I run Hankook RS4s, and for autocross I put a set of Bridgestone RE71s on the front, mainly because they have better low temp grip and bit better lateral transition characteristics for the severe maneuvers in autocross than the RS4s.

Thank you.
Looks like mixing racing tyres with ultra premium road tires is not a good idea. Racing tyres also seem trickier to deal with on every day driving . I think I'll stick with the recommended tire dimensions to keep it legal an choose some ultra premium road tyres, but I want a bit less rated grip frot the rear as the tire width will make up for it, like i.e., pirelli pz4 front and continental premium contact 6 for rear.
My driving style is to enter slower into the corner and then push it trough until it starts to loose grip. As I start to know the track I gradually reduce the braking time and brake later, but I still like not to enter at the very limit of grip.

Using racing tyres would make the car faster through the corners and I think it would be a totally new experience, but it would also add more wear to the car components and my car is not equiped for that, plus it is starting to get old now.

The foremost issue with racing compounds for street driving is the lower temperature limit. The rubber compounds used to give high grip when hot also tend to get very hard and stiff at temperatures near freezing (0C). Here's what's on Tire Racks site about the R888 and cold weather:

Proxes R888 radials are not intended to be driven in near-freezing temperatures, through snow or on ice. It's also essential these tires be stored indoors at temperatures maintained above 15 degrees F

All the rage around here these days are the Nankang NS2R. Decent reviews overall and the tire shops around here are calling them the best value for a decent tire.

I've done only a few track days with mine and so far am quite pleased (factory size and I only use them for the track). My buddy drove his on the street as well and found them fine, just a little loud if I recall.

The foremost issue with racing compounds for street driving is the lower temperature limit. The rubber compounds used to give high grip when hot also tend to get very hard and stiff at temperatures near freezing (0C). Here's what's on Tire Racks site about the R888 and cold weather:

Proxes R888 radials are not intended to be driven in near-freezing temperatures, through snow or on ice. It's also essential these tires be stored indoors at temperatures maintained above 15 degrees F

I have a set winter tyres and rims for the cold season as it is illegal here to drive on snow or ice on anything else. When the temperature in the morning starts to fall bellow 10C, I switch to winter tyres.

All the rage around here these days are the Nankang NS2R. Decent reviews overall and the tire shops around here are calling them the best value for a decent tire.

I've done only a few track days with mine and so far am quite pleased (factory size and I only use them for the track). My buddy drove his on the street as well and found them fine, just a little loud if I recall.

Price was awesome.

Nankang may be an option as they are relatively cheaper. If I were to choose semi slicks, I have few options here, these, toyo and I've found some hankooks r-s3's, 245/40/R18.

If this is your first track day, you do NOT want R-Comp Tires. They are not forgiving enough. Street tires will give you ample warning on loss of traction. Besides, you will be busy with a million other, non-tire, related issues.

Before you worry about tires, focus on you. If you want something to do, be sure you have 50%+ on your brake pads and fresh brake fluid (preferably a good DOT 4 like ATE 200).

If this is your first track day, you do NOT want R-Comp Tires. They are not forgiving enough. Street tires will give you ample warning on loss of traction. Besides, you will be busy with a million other, non-tire, related issues.

Before you worry about tires, focus on you. If you want something to do, be sure you have 50%+ on your brake pads and fresh brake fluid (preferably a good DOT 4 like ATE 200).

Thanks. Yes, I'll take your advice and go initially with street tires.
Now I'll have to choose what size to go with. I would try 235 or 245 35 r18, 8" rims front and 255 35 r18, 8.5 rear. I wonder what offset would be required on the front.

Thanks. Yes, I'll take your advice and go initially with street tires.
Now I'll have to choose what size to go with. I would try 235 or 245 35 r18, 8" rims front and 255 35 r18, 8.5 rear. I wonder what offset would be required on the front.

Thanks. Yes, I'll take your advice and go initially with street tires.
Now I'll have to choose what size to go with. I would try 235 or 245 35 r18, 8" rims front and 255 35 r18, 8.5 rear. I wonder what offset would be required on the front.

I've run 255/35 x 18 and 245/40 x 18 on stock 8" Style 108 wheels in the front. OEM offset is 47mm. I currently have 245/40 x 18 on FlowOne wheels with 42mm offset. All have run fine with no issues. On the rear I've run 275/35 x 18 and 265/35 x 18 on the stock Style 108 (8 1/2" wide, 50mm offset) and I'm currently running 265/35 x 18 on FlowOne wheels with 40mm offset, again with no issues.
The 255/35 were a bit of a pinch fit on the 8" rim which is why I dropped back to the 245/40, but they had no rub issues. Same with the 275/35 on the rear. No rub issues, but I dropped down to the 265 to reduce the amount of pinch. You should have no issues with 245/35 x 18 for the front.
I'm getting ready to buy another pair of the 8.5" FlowOne wheels and mount 255/35 x 18 on them and those will be my fronts for track days. I'll stick with the 265/35 on the rear for the rear track day set.

The stock 225/45 tires on the front are purposely downsized in width to assure you get no response other than under steer and they also act as a speed limiter. That's fine for the street, but once you get on the track and start to push the limits a little bit you quickly find the car doesn't want to turn and cornering gets very frustrating. With the sizes I've referenced above you'll still get sufficient under steer to assure that's the dominant characteristic of the car, but it won't be so severe that you can't make the car go quickly.

I've got the tyres, 265/35R18 and 255/35R18, Michelin pilot sport 4, and now I'm waiting for the rims to come. I've ordered a set of 4 Japan Racing rims 8.5" wide, et35, but for the 265 tyres I think I should've chosen 9" or 9.5 wide rims. Will these 8.5 be alright?
Thanks

I've got the tyres, 265/35R18 and 255/35R18, Michelin pilot sport 4, and now I'm waiting for the rims to come. I've ordered a set of 4 Japan Racing rims 8.5" wide, et35, but for the 265 tyres I think I should've chosen 9" or 9.5 wide rims. Will these 8.5 be alright?
Thanks

Both those sizes will be fine on the 8.5 inch rim. On the standard staggered wheel set 255/35 x 18 on 8.5" rim is OEM standard. You're only adding 10 mm with the 265/35. I've run 275/35 x 18 on the 8.5" rim with no issues. With the ET35 wheels you might even be able to run the 265/35 on the front. Clearance between the sidewall of the tire and the body of the strut is the limiting factor. With 255/35 tires on ET40 wheels I've got about 5mm clearance. With your ET35 wheels you should get enough added clearance.

I've mounted the wheels and they fit well enough. They touch a bit on the plastic wheel arches covers, nothing major, when turning the steering wheel to the end of travel but the original winter ones did that even a bit worse. Here are some quickly taken photos.

I've mounted the wheels and they fit well enough. They touch a bit on the plastic wheel arches covers, nothing major, when turning the steering wheel to the end of travel but the original winter ones did that even a bit worse. Here are some quickly taken photos.

Depending on where the rub is on the plastic fender liners, you may be able to fix that with a heat gun, a wooden spoon, and a spray bottle of water.

Thanks for the advice. The wheels barely touch the plastic covers only at some angles and I can go like that, no problem, the rub is minimal, but if I want it to be perfect I can try that.
Anyway these wheels with these dimensions look very nice on the car and it handles very well. I didn't test it on the track yet.

Here is a video on a racetrack, but I couldn't go too fast since it was my first time on a fast race track.

Regards how the wheels fit in the well, I still have some rubbing issues. The wheels have grinded a some portions of the covers where it is not possible to push them in further. That is also because the bumper stays a bit lower because of sole plastic side holders which I will replace later on. Also it happens only when I cross over bumps or small ridges. Speed bumpers for example are no problem. I've repaired the covers using a copper sheet that can resist better to abrasion and also replace two plastic rivets that were melting and also didn't provide enough support, with metal screws and standard oem metal clips. It held well enough up until now.

Here is a video on a racetrack, but I couldn't go too fast since it was my first time on a fast race track.

Regards how the wheels fit in the well, I still have some rubbing issues. The wheels have grinded a some portions of the covers where it is not possible to push them in further. That is also because the bumper stays a bit lower because of sole plastic side holders which I will replace later on. Also it happens only when I cross over bumps or small ridges. Speed bumpers for example are no problem. I've repaired the covers using a copper sheet that can resist better to abrasion and also replace two plastic rivets that were melting and also didn't provide enough support, with metal screws and standard oem metal clips. It held well enough up until now.

Wow! Rules for being on track seem to be far more lax there than here! Here in the US first time out you have an instructor in the right hand seat. Also if one of the marshals saw us fidgeting with our phone on track we'd get black flagged, pulled off the course and get a lecture.

I was only trying to press a single button on that phone, but it wouldn't work.
Before this I went to a shorter track and they have presented me the track and did few laps with an instructor, but here they didn't ask me too many questions. Anyway, I'm sure they will fix these issues in the future since the circuit was opened only few years ago and this is the first high speed circuit.

I've switched to Fr 225 by R 255 due to regulations. ***x1f642; So...moved the 255 from front to rear. The 255's were a bit large and interfering with the liners and bumper support. The 225 have good clearance although the rims ET is 34. But the rims are all 8.5" and the 225 tires look a bit bloated. However, it says 225is minimum and 235,245 is ideal size so they should be alright.
The grip seems fine through the corners as well, even though there should be more understeering perhaps. Understeering doesn't mean it feels like a FWD. I did some si racing and now have some idea of what how it affects the dynamics. It's easier to keep it under control.